Waste water containing nitrate ions such as ammonium nitrate or sodium nitrate is generated in various kinds of industries including non-ferrous metal refining and petrochemical industry. Such waste water containing nitrate ions has been treated by conventional methods known in the art including activated sludge process and thermal decomposition method.
The activated sludge process has been put into practice in sewage treatment plants. In this method, appropriate activation of the function of microorganisms requires control of the nitrogen compound concentration in the incoming waste water containing nitrate ions not to exceed 0.3% by weight in terms of ammonium nitrate. Thus, treatment of a waste water containing high concentration of nitrate ions necessitates a large amount of dilution water, vast area for treatment, expensive plants, and considerable treatment cost. Moreover, treatment of a large volume of sludge which has been generated poses another problem.
Ammonium nitrate, as an example of a nitrate ion source, is known to be thermally decomposed in accordance with the following formulae (Chemical Handbook (Kagaku Binran), Application, Third Edition, p115):
NH.sub.4 NO.sub.3 .fwdarw. NH.sub.3 + HNO.sub.3 -41 kcal/mol (180.degree. C.) (1) NH.sub.4 NO.sub.3 .fwdarw. N.sub.2 O + 2H.sub.2 O +10 kcal/mol (250.degree. C.) (2) 2NH.sub.4 NO.sub.3 .fwdarw. 2N.sub.2 + 4H.sub.2 O + O.sub.2 +28 kcal/mol (300.degree. C.) (3) 4NH.sub.4 NO.sub.3 .fwdarw. 2NO.sub.2 + 8H.sub.2 O + 3N.sub.2 +27 kcal/mol (4)
Heating a waste water containing ammonium nitrate induces decomposition of ammonium nitrate through the reactions represented by the formulae (1) and (2) stepwise depending on the temperature of the ammonium nitrate, thereby generating HNO.sub.3 and N.sub.2 O, which cause formation of NOx. When the temperature for decomposition is lower, however, the reaction of the formula (1) proceeds in the reverse direction, resulting in recombined ammonium nitrate. In order to suppress the generation of the recombined ammonium nitrate and to decrease the decomposition gases, the temperature of ammonium nitrate should be rapidly raised to 300.degree. C. or higher so that the reaction represented by the formula (3) proceeds. Thus, the conventional thermal decomposition method requires supply of the waste water by spraying into a decomposition furnace, while the furnace is kept at a temperature of not lower than 1000.degree. C. to facilitate increase in temperature of the waste water. However, at a temperature of as high as not lower than 1000.degree. C., nitrogen gas and oxygen react to form thermal NOx.
For overcoming such drawbacks, there has been proposed a two-stage decomposition method including the steps of decomposing ammonium nitrate at a lower temperature, and then further decomposing the resulting decomposition gas at a higher temperature.
One two-stage decomposition method known in the art follows the steps of spraying a concentrated ammonium nitrate solution into a heating furnace held at 180 to 210.degree. C. to cause thermal decomposition, and then introducing the resulting decomposition gas into a decomposition furnace previously heated to 600 to 1000.degree. C. for further decomposition (JP-52-22751A). This method, however, requires a decomposition furnace equipment that is resistant to a temperature of as high as 1000.degree. C., and generates an exhaust gas containing as much as 0.4% NOx.
Another two-stage decomposition method known in the art for treatment of an ammonium nitrate waste water containing radioactive residue includes the steps of thermal-decomposing the waste water contained in a vessel in a primary decomposition furnace previously heated to 250 to 310.degree. C., and then further decomposing the resulting recombined ammonium nitrate in a secondary decomposition furnace previously heated to 350 to 600.degree. C. (JP-62-52277B). However, this method results in undesired generation of as much as 1.9% NOx since the decomposition temperature in the secondary decomposition furnace is maintained at a temperature of as low as 350 to 600.degree. C. for preventing gassification and dispersion of the radioactive residue.